SummaryWe tested the hypothesis that agonistic behavior and interference competition induce species replacements in freshwater decapods. Our model organisms were two crayfish (the indigenous Austropotamobius italicus and the non-indigenous and invasive Procambarus clarkii) and the indigenous river crab Potamon fluviatile. A first experiment was aimed at analyzing the agonistic behavior of similarly-sized males in pairs of the three species combinations. Records were taken for an hour per day during five consecutive days of combats. Results showed that P. fluviatile was dominant over the two crayfish species and P. clarkii over A. italicus, as confirmed by their field distribution. In nature, the non-indigenous species might even reach higher levels of dominance over A. italicus. In fact, both the larger body size and the 'stronger' chelae of P. clarkii can induce asymmetries in fighting ability. Pairs composed of the non-indigenous crayfish and either A. italicus or P. fluviatile did not form stable hierarchies, possibly due to the failure of status recognition. In a second set of experiments, the agonistic behavior of the three species combinations was studied in the presence of either food (earthworms) or an artificial shelter (a 10 cm-long piece of a PVC pipe). As expected, resources influenced fighting and dominance translated into a differential capability to compete. In a third experiment, in which we measured shelter use by each species in a non-competitive context, shelters were more extensively occupied by A. italicus (which is dependent on natural crevices as hiding places) than by the other two species (which usually dig burrows). We expected that Potamon and Procambarus would gain less from occupying the offered shelter than Austropotamobius, and therefore their defense should be less vigorous. To the contrary, the presence of a rival strengthened the attraction to the shelter of these two species, in particular when river crabs were opposed to A. italicus.

Juvenile and immature crayfish are more difficult to find than adults and in consequence rather little is known about their habitat preferences. Smaller crayfish are able to inhabit different areas within the stream compared to adult crayfish. Whereas adult crayfish occur under stones and cobble in the river, juveniles and small individuals may be found along the margins, among rootlets and vegetation. A survey was carried at two sites in the Liffey catchment (Ireland) to assess habitat preference of crayfish according to size. A Surber sampler was used to collect crayfish in five types of habitat present at these sites. Each site presented a different distribution of crayfish amongst the habitat types. At Kilbride bridge, crayfish preferred cobble to pebbles and submerged and emergent macrophytes, while at Carragh bridge, crayfish were more dispersed throughout the habitat types. At both sites, juveniles and one year old crayfish were found in a variety of habitat types, whereas adults were only found in cobble and pebbles. The discrepancy in crayfish densities between the two sites can probably be explained by characteristics, such as depth and flow rate, that were not taken into account in the categorisation of the habitats. Key-words:Austropotamobius pallipes, juvenile, habitat. HABITAT PRÉFÉRENTIEL D'AUSTROPOTAMOBIUS PALLIPES SELON LA TAILLE DANS UN RIVIÈRE EN IRLANDE RÉSUMÉL'utilisation de l'habitat par les écrevisses juvéniles est relativement peu connue en raison de leur petite taille les rendant plus difficiles à échantillonner que les adultes. Les écrevisses de petite taille peuvent habiter des habitats plus diversifiés que les adultes. Les adultes trouvent refuge sous roches et cailloux alors que les écrevisses plus petites peuvent être trouvées également le long des berges, dans les radicelles ou dans la végétation aquatique. Cette étude a été menée sur deux sites dans le bassin du fleuve Liffey dans le but de déterminer la préférence en matière d'habitat des écrevisses à pattes blanches en fonction de leur taille. Un filet Surber a été utilisé pour l'échantillonnage des écrevisses dans cinq types de substrats. A Kilbride bridge, les écrevisses ont surtout été